APU on for takeoff?

[FL410]

Is it common practice to leave it on in case of an engine failure and having the other bus fail?

Curious. [img]/ubbthreads/images/graemlins/cool.gif[/img]

[flyover]

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Is it common practice to leave it on in case of an engine failure and having the other bus fail?

Curious. [img]/ubbthreads/images/graemlins/cool.gif[/img]

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Not common, especially with newer aircraft types.

With the older generation jets, generators were notoriously inadequate to carry the load if one failed. So in some aircraft, like the DC-9, a captain might want to leave the APU up and available if the weather was low or threatening.

Newer generation jets have ample generator power so that one can carry the load if needed. So keeping the APU up and running is seldom considered now.

[MikeD]

[ QUOTE ]
[ QUOTE ]
Is it common practice to leave it on in case of an engine failure and having the other bus fail?

Curious. [img]/ubbthreads/images/graemlins/cool.gif[/img]

[/ QUOTE ]

Not common, especially with newer aircraft types.

With the older generation jets, generators were notoriously inadequate to carry the load if one failed. So in some aircraft, like the DC-9, a captain might want to leave the APU up and available if the weather was low or threatening.

Newer generation jets have ample generator power so that one can carry the load if needed. So keeping the APU up and running is seldom considered now.

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Agree. For us, in the A-10, the APU was shutoff prior taxi or takeoff. It was only used on the ground for engine start and avionics cooling if the OAT was 75F+. Other than that, it was an extra generator and extra hydraulic pump that could be started if needed. In the 117, the APU stays on all the time, from start to shutdown. It's neither a generator nor a hydraulic pump, merely an air source; but we're required to keep it up and running all the time.

[Mr_Creepy]

We took off with the APU on in the CRJ as a regular procedure.

[FalconCapt]

[ QUOTE ]
Is it common practice to leave it on in case of an engine failure and having the other bus fail?

Curious. [img]/ubbthreads/images/graemlins/cool.gif[/img]

[/ QUOTE ]

In the Falcon 50/900's if you take off with the APU running, it shuts itself off as soon as weight is off the main wheels... Or so I have heard... [img]/ubbthreads/images/graemlins/tongue.gif[/img]

[flyover]

[ QUOTE ]
We took off with the APU on in the CRJ as a regular procedure.

[/ QUOTE ]

Curious what the reasoning is.

[mpenguin1]

I will ask some of the older instructors around the office, I know that on the P3 Orion & the
A3 Skywarrior, APU was off after the #2 engine was started. I jumpeated on the 747 enough
times, APU was off as well. Will check, could be interesting to see what I find out.

[FlyChicaga]

We leave the APU running and supplying the packs for all departures, unless we need engine anti-ice. Then we turn the packs off, open the engine bleeds, and close off the APU bleed... but still keep it running. For flights less than 45 minutes, we'll keep it running the entire flight. It's a quick generator and bleed source should an engine fail, generator drop offline, bleed fail closed, etc.

Having the APU running for all depatures makes each one a "bleeds off" takeoff, therefore giving better takeoff performance. The only thing our APU can't supply is anti-ice.

[JAM]

[ QUOTE ]
[ QUOTE ]
We took off with the APU on in the CRJ as a regular procedure.

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Curious what the reasoning is.

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Passenger comfort, redundant generator, safety. Probably more reasons that I am not thinking of at the moment. Or is it just to give us more buttons to push in the climb checklist?

[FL410]

Thanks guys, that was great. [img]/ubbthreads/images/graemlins/grin.gif[/img]

[FL270]

CHQ uses a different procedure than XJT although it is the same airplane. We do not have APUs on for takeoff and landing as a general rule. Certain MEL items may require it but that is basically it.

CHQ, XJT, and Eagle are the three largest ERJ operators in the world, and all three have different APU procedures. Pretty much just operator preference. Embraer didn't make any requirements in this regard ... other than, as Matt pointed out, the Embraer's APU can't supply bleed air to the anti-icing systems. That is a limitation.

[C650CPT]

Here is my favorite Aviation answer : It just depends.

On our Citation 650 we will use the APU for normal preflight checks ( electrical and hydraulic ) and for getting the avioncs up and running. If it is hot or cold it also supplies us with enviromental air as appropriate. If it really hot we leave it on for takeoff and initial climb as it really helps cool the airplane, plus we can then depart with the engine bleeds off which gives better hot/high performance. If I am departing in low IFR conditions, since it is already running I am inclined to leave it on for the redunancy of electrical supply should it be needed. In a "normal" engine failure you may not loose the other bus ... depends on the design of the system. Most are tied together with bus tie or current limiters, you would need both a failure of a power supply and a same side current limiter before you would notice a bus problem.

[NJA_Capt]

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Not common, especially with newer aircraft types.

Newer generation jets have ample generator power so that one can carry the load if needed. So keeping the APU up and running is seldom considered now.

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Not always true...

We use the APU in the Citation X for takeoff and landing. It is certified to operate to FL310, but we turn it off above FL180. We operate it continuously below FL180.

We operate the Citation Excel the same way. APU on below FL180.

[Mr_Creepy]

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We took off with the APU on in the CRJ as a regular procedure.

[/ QUOTE ]

Curious what the reasoning is.

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Two Words. Air Conditioning!

[N9103M]

[ QUOTE ]
[ QUOTE ]
We took off with the APU on in the CRJ as a regular procedure.

[/ QUOTE ]

Curious what the reasoning is.

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When I flew the Reset Jet, normal procedure (at our company) was to shut down the APU sometime during the taxi out to the runway and transfer the bleeds over to the engines. But if you needed wing and cowl anti-ice you took off with the bleeds on the APU, or else you took too much of a performance hit. But the APU was then shut off in the climb check.

On arrival, you'd keep it off unless again, you needed both wing and cowl anti-icing. But much like on some airplanes, using just the bleeds on the engines on the ground provided almost no A/C. It was absoulutely terrible to be stuck with a deferred APU and a long ground stop. [img]/ubbthreads/images/graemlins/banghead.gif[/img]

[deskjockey]

Actually, in the P-3 Orion you would start #2 off the APU, then start #3 and taxi, or start all 4. Shutting down the APU after starting 2 would leave you single generator.

In flight the APU is used only as backup. Some engineers would start it prior to landing so that they didn't have to do it on the ground. Just personal preference. With 4 engines you don't really need a backup generator all ready on line.